It has been known to produce viscous crude oils in reservoirs by drilling vertical wells into the producing zone and then injecting steam into the viscous crude to increase its mobility and reduce its viscosity. This steam injection has been done in several different ways. In one technique producing wells in the reservoir can be cyclically steamed by injecting steam down a vertical well into the production zone for a relatively short period of time. The well is then placed on production for a relatively longer period of time and this cycle repeated until the production becomes unprofitable.
Another technique which has been used to produce viscous crude reservoirs is to drill vertical wells in a geometrical pattern into the production zone and to designate certain of these wells as injection wells. Steam is then continuously injected into the production zone via the injection wells in an attempt to drive the steam and its heat to move the viscous crude oil to the other vertical producing wells in the geometrical array.
In the initial development of a reservoir of viscous crude these described methods have worked well. Over time however, the steam tends to congregate in the upper portion of the producing zone. This, of course, does not cause heating of the viscous crude in the lower portion of the producing zone. The heavy crude saturated lower portion of the producing zone is not depleted as the high viscosity of the crude prevents its easy migration to the well bores of the producing wells. Thus large quantities of potentially producible crude oil can become otherwise not recoverable.